In recent years, external I/Fs (InterFaces) such as USB or IEEE 1394 are mounted on an electronic device represented by a personal computer. The external I/Fs decrease the number of signal lines (bus width) and transmit high-speed signals having pulse widths corresponding to a frequency of several hundred MHz to ensure the band width. The external I/Fs use low-amplitude differential signals as low as about several ten 10 mV to ensure such high speed and noise-resistant ability. On a printed wiring board in the device, the high-speed I/F signals are transmitted by signal lines which are matched to the differential impedances complying the respective I/F standards. This aims at avoiding decrease in signal quality caused by the reflection or attenuation of the differential signals on the printed wiring board.
The differential signal wiring patterns require at least two dedicated GND (Ground) wiring patterns that are differential-impedance matched. Accordingly, the two differential signal wiring patterns and two GND wiring patterns must be extended in one path to increase the wiring pattern width of the signal group.
This will be exemplified by a case wherein a wiring pattern having a USB differential impedance of 90 Ω is extended in the first layer, the thickness of the insulating layer between the first layer and the second GND layer is 0.2 mm, and the dielectric constant is 4.5. When the width of this wiring pattern is calculated by a commercially available transmission path simulator, SPACE/LINE/SPACE/LINE/SPACE=300 μm+175 μm+125 μm+175 μm+300 μm=1,075 μm is obtained. In other words, the entire width of the wiring pattern group including the clearances at the two ends is about 1.0 mm. The minimal design value of the substrate at this time is calculated as L/S=0.125/0.125 mm.
To extend such a wiring pattern from BGA signal terminals assigned with signals at a 1.0-mm pitch without impedance mismatching, the differential signals must be assigned to the balls on the first to third arrays counted from the outer side.
If the differential signals are to be assigned to the balls from the fourth array, through hole lands on the first to third arrays which result from wiring pattern extension of other signals serve as an obstacle, and a 1.0-mm pitch signal line group cannot be arranged. For example, when the through hole land diameter is 0.6 mm and the through hole pitch is 1.0 mm, the distance between the through hole lands is 0.4 mm, and a 1.0-mm pitch signal line group cannot be arranged.
Japanese Patent Laid-Open No. 2000-349192 discloses a printed wiring board on which a BGA (Ball Grid Array) package is mounted. According to the method proposed by Japanese Patent Laid-Open No. 2000-349192, high-speed differential signals are assigned to the outermost pins to facilitate signal wiring pattern extension and avoid mutual interference with other signals.
In general, the length of a signal wiring pattern starts to adversely affect the waveform by causing reflection or the like where the time required by the signal to reciprocate on the path exceeds the signal rise time. Usually, the rise time of a trapezoidal wave is about 5% the signal period.
For example, when the frequency is 100 MHz, the period is 10 nsec, and the rise time is 0.5 nsec. When the region where the time required by the signal to reciprocate on the path exceeds the signal rise time is converted using a signal transmission speed of 0.006 nsec/mm on a general FR4 substrate, the corresponding reciprocal length is 0.5/0.006=83.3 mm. The one-way length is 83.3/2=about 42 mm. Namely, impedance mismatching of a path with a length of 42 mm or more largely, adversely affects the waveform quality.
The wiring pattern density on an interposer substrate which forms a BGA package is high unlike on the printed wiring board, and accordingly it is very difficult to form an impedance-matched wiring pattern on the interposer substrate. The length of one side of a generally widely used BGA package is 50 mm or less, and the maximal distance from the center is about 35 mm (a length half the diagonal line). Hence, regarding the wiring pattern length that can be an issue, the wiring pattern on the interposer substrate is sufficiently short, and conventionally no problem arises as far as the wiring pattern length on the printed wiring board is considered.
As the signal speed increases, however, the frequency increases, and a signal having a frequency corresponding to 500 MHz or more cannot but be employed.
When the calculation described above is done for the frequency of 500 MHz, the wiring pattern length that causes a problem in signal reflection is 8 mm. Namely, impedance mismatching of a path with a length of 8 mm or more largely adversely affects the waveform quality.
As the wiring pattern width on the interposer which does not conventionally cause any problem is becoming to largely, adversely affect the waveform quality, the high-speed differential signal pins cannot but be assigned on the inner side of the BGA. In this case, however, it is difficult to extend a wiring pattern from a terminal on the printed wiring board. Accordingly, to extend the wiring pattern while maintaining the waveform quality, an expensive IVH circuit board or build-up wiring board must be used, which is a significant problem.